A sodium ion battery is a battery such that an Na ion moves between a cathode and an anode. Na exists so abundantly as compared with Li that the sodium ion battery has the advantage that lower costs are easily intended as compared with a lithium ion battery. Generally, the sodium ion battery has a cathode active material layer containing a cathode active material, an anode active material layer containing an anode active material, and an electrolyte layer formed between the cathode active material layer and the anode active material layer.
Na2Ti6O13 is known as the anode active material used for the sodium ion battery. For example, the sodium ion battery using Na2TiO13 for the anode active material is disclosed in N. D. Trinh et al., “Synthesis, Characterization and Electrochemical Studies of Active Materials for Sodium Ion Batteries”, ECS Transactions, 35 (32) 91-98 (2011). Also, although not the sodium ion battery, the lithium ion battery using Na2Ti6O13 for the anode active material is disclosed in J. C. Perez-Flores et al., “On the Mechanism of Lithium Insertion into A2Ti6O13 (A=Na, Li)”, ECS Transactions, 41 (41) 195-206 (2012). The same description is given also in Prior Art of Japanese Patent Application Publication (JP-A) No. 2009-117259. Also, the sodium ion battery using lithium titanate (Li4Ti5O12) for the anode active material is disclosed in JP-A No. 2011-049126. Also, it is disclosed in JP-A No. 2007-048682 that an active material and a carbon material are composited by a ball mill.